1. Field of the Invention
This invention relates to a process and apparatus for recycling waste cellulosic material to produce a bleachable cellulosic fiber pulp. The invention is especially concerned with recycling waste corrugated paperboard.
2. Description of Prior Art
Corrugated containers are normally produced with linerboard and a corrugating medium. High yield softwood kraft is usually the pulp used to produce linerboard, while high yield semi-chem hardwood is the normal pulp for making corrugating medium. The common characteristic of these pulps is their high lignin content. Linerboard pulps contain about 10% lignin, corresponding to a kappa of about 70. Corrugating medium pulps have about 18% lignin, corresponding to about 120 kappa. The lignin content of corrugated containers is therefore of the order of 12-14%, corresponding to about 90 kappa (% lignin.apprxeq.kappa.times.0.15).
Old corrugated containers (OCC) have traditionally been considered paper waste, which usually ends up in a landfill site. The extent of recycling of OCC for making recycled linerboard and corrugating medium paper products is currently at 40% at most. The strength of old corrugated containers, due to their usage and recycled fiber content, is known to be very low, corresponding to a viscosity of about 28 to 32 by TAPPI T-230 Method. For reference purposes, the TAPPI viscosities of virgin linerboard and corrugating medium pulps are as high as 50 to 60.
In order to increase the percentage of OCC which can be recycled and used in the manufacture of high quality linerboard, a method of delignifying OCC with oxygen in the presence of an alkali has been proposed by A. de Ruvo, P. A. Farnstrand and N. Hagen in an article presented at the 1985 TAPPI pulping conference. The proposed oxygen delignification of OCC was said to result in substantial loss of the OCC viscosity. As illustrated in this publication, when the kappa of OCC was reduced from 70 to 28, with the viscosity of OCC was a low as 265 by SCAN C15:62 method, which is equivalent to 3.0 by TAPPI T-230 method.
In order to economically bleach high kappa virgin pulp rejects such as knots, shires, chops and partially cooked chips, U.S. Pat. No. 4,435,249 proposes to delignify these pulp rejects with oxygen in the presence of an alkali chemical. The objective was to reduce their kappa from about 90 to about 20 to 30, at which kappa they can be economically bleached, while meeting environmental regulations. The bleaching of these low kappa pulps is designed to substantially remove the residual lignin content and to brighten the pulp, which can then be used to produce wood-free (by which is meant lignin-free) printing and writing paper grades.
Oxygen delignification, when used on unbleached pulps to reduce their lignin fraction, is known to simultaneously degrade the cellulose fraction. This undesirable cellulose degradation is also known to be responsible for the observed loss in the pulp viscosity and has usually dictated how low in kappa the pulp can be delignified while maintaining a minimum viscosity/strength for making papers.
The strength of fully bleached pulps, used on commercial paper machines to produce these fine paper grades has to be very high. For these pulps, a TAPPI viscosity of at least 10 is usually the minimum requirement to operate the paper machine with good efficiency and to produce papers with satisfactory strength.
The bleaching of softwood kraft pulps usually commences with unbleached pulps with a kappa of 30 to 35 range. In order to reduce the bleaching cost, U.S. Pat. No. 4,946,556 proposed the use of oxygen delignification to reduce the kappa of unbleached pulps to less than 10. Similarly, to minimize the viscosity loss associated with the delignification, oxygen treatment in multistages, together with countercurrent pulp washing between oxygen stages was proposed as a necessary requirement. More viscosity protection was also found when a chelating agent such as EDTA was added to the countercurrent wash liquid and/or pulps were pretreated with a chelating agent such as DTPA. Interstage pulp washing, as taught in the U.S. patent, has been known to substantially increase the complexity and cost of carrying out the oxygen delignification, for example more than double the capital expenses and operating cost.